Radiation sensors have long been relied on to determine if a hopper or vessel has been filled with material to a specified level. In a widely used approach, the sensors utilize a radiation source which produces a beam of gamma radiation at the specified level and a radiation detector, such as a Geiger-Mueller tube, which is aligned with the beam. The absence of material at the level of the beam allows relatively strong radiation flux to reach the detector while continuous presence of the product at the level of the beam attenuates the flux by an amount sufficient to interrupt detection of the beam by the Geiger-Mueller tube.
To date, the effectiveness of using Geiger-Mueller tube radiation sensors to measure material levels has been limited because, according to prior art techniques, Geiger-Mueller tubes may not be separated more than a few feet from the measurement electronics used therewith. This is because Geiger-Mueller tubes produce very fast pulse outputs having a repetition rate directly proportional to the amount of radiation sensed by the tube. If a cable is introduced between the tube and the measurement electronics, the cable will modify the pulse by both reducing the strength of the pulse and changing the shape of the pulse. The reduction in strength is a result of the cable's electrical resistance and dielectric loss while the modification of shape is due to the cable's inherent capacitance. The foregoing effects increase in magnitude with the length of the cable and interfere drastically with the operation of measurement circuits currently used. The circuitry presently used is designed to accept the very short output pulses normally associated with Geiger-Mueller tubes and will not operate with the much longer, integrated pulse which occurs at the end of a long cable run. Consequently, the electronic circuitry currently available must be located in close proximity to the tube to produce a usable output.
A primary disadvantage of having the electronic circuitry for each individual Geiger-Mueller tube is close proximity therewith is that the circuitry requires its own power supply. This disadvantage is compounded when there are a plurality of detectors using Geiger-Mueller tubes dispersed over a wide area. As is readily seen, the more detectors one has, the more expensive and cumbersome such a system may become. Power for each Geiger-Mueller tube must somehow be delivered to the site of the tube and matched with the electronics associated with the tube. In an industrial application such as monitoring material levels in vessels or hoppers, power may or may not be initially available at the sites of the vessels or hoppers or if available, the power may not be suitable for the electronics. In either case each Geiger-Mueller counter or closely associated group of Geiger-Mueller counters must have power supply facilities provided. If the power is dispensed from a central station auxiliary cables must be provided which is, of course, a relatively cumbersome and expensive approach involving many design, installation and maintenance problems. With prior art approaches amplification of and modification of the signals produced by measurement electronics at the site of measurement is necessary in order to coordinate signals from detectors with the needs of a central control. These problems have discouraged the use of radiation type level detectors for monitoring and controlling widely dispersed hopper or vessel filling operations. This has resulted in industries relying on other approaches which are either labor intensive or not otherwise entirely satisfactory because the advantages of radiation-type sensing are not utilized.
The following patents are indicative of radiation-type sensors currently available for monitoring the level of material in an enclosure of some type:
______________________________________ 2,323,128 Hair 2,713,124 Graham 2,972,050 Allen 3,389,250 Clemens 3,560,801 McCarther 3,668,392 Bajek et al 3,944,830 Dissing ______________________________________
None of the above cited patents disclose the concept of a single panel which controls a plurality of detectors monitoring separate widely dispersed hoppers or vessels.
Considering the above listed patents individually, U.S. Pat. No. 3,389,250 discloses the use of Geiger-Mueller tubes for sensing the level of liquid in a tank, however, there is no discussion or appreciation of the concept of monitoring levels of fluid in the tank from a remote location. Furthermore, the power supply is located in proximity to the tank.
U.S. Pat. No. 2,713,124 is concerned with monitoring the level of solid material in a vessel with a Geiger-Mueller tube which is connected to a recorder spaced from the Geiger-Mueller tube by a "substantial distance." This "substantial distance" is however, only the distance from the Geiger-Mueller tube to the bottom of the tank and a preamplifier is disposed adjacent to the Geiger-Mueller tube in order to boost the signal from the tube so as to render the signal detectable by another amplifier associated with the recorder. The use of a preamplifier in proximity with each Geiger-Mueller tube decreases the practicality of using Geiger-Mueller tubes as detectors for a centrally monitored system wherein the Geiger-Mueller tubes are both dispersed and widely spaced.
U.S. Pat. No. 2,972,050 discloses a detector connected to a recorder by a relatively long cable, however, the detector is not specified as a Geiger-Mueller tube detector and an amplifier associated therewith is disclosed as being either in proximity to the detector or in proximity to the recorder. Furthermore, U.S. Pat. No. 2,972,050 does not suggest any specific arrangement for positioning the amplifier remotely from the detector and adjacent to the recorder.
U.S. Pat. Nos. 2,323,128 and 3,668,392 each disclose radiation type detectors which are connected to monitoring means by cables, however, the particular problem of having very long cables is not dealt with or discussed.
U.S. Pat. Nos. 3,560,801 and 3,944,830 are exemplary of prior art disclosures concerned with the electronics associated with radiation type detectors, however, these patents do not deal with the problems of remotely monitoring a plurality of detectors located at dispersed and distant stations.
As is readily seen by the aforementioned considerations, there is a need for some type of economical method or apparatus for remotely monitoring Geiger-Mueller tube detectors so that a plurality of these detectors can be monitored from a central station even if the detectors are widely dispersed.